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Hydralazine: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on May 24, 2022.

1. How it works

  • Hydralazine is a vasodilator - this means it relaxes smooth muscle inside blood vessels, making the blood vessels wider, and allowing more blood to flow through. This reduces how much force the heart has to exert to pump blood through the blood vessels, and reduces blood pressure.
  • Experts aren't sure exactly how hydralazine does this but suggest it may alter calcium movements through muscle which affects how much the muscle contracts.
  • Hydralazine belongs to the class of medicines known as vasodilators.

2. Upsides

  • May be used for the treatment of high blood pressure (BP), either alone or in combination with other agents.
  • May be given in combination with isosorbide dinitrate to treat heart failure in self-identified black patients in addition to standard therapies to improve survival, decrease hospitalization rates, and improve patient well-being.
  • Available as oral tablets or in an injectable form. Usually, oral tablets are given but the injectable form of hydralazine may be used by healthcare providers when there is an urgent need to lower blood pressure or if the oral tablets cannot be given. 20–25 mg of IV hydralazine hydrochloride is approximately equal to 75–100 mg of oral hydralazine hydrochloride.
  • Hydralazine also improves blood flow to the outer extremities (such as the fingers and toes), increases heart rate and the volume of blood pumped with each heartbeat, and overall heart performance.
  • More effective at reducing diastolic (the bottom number of a BP reading) rather than systolic (the top number of a BP reading) blood pressure.
  • Less likely to cause a drop in blood pressure on standing (postural hypotension) as it preferentially targets arteries rather than veins.
  • Has been given to children; however, its safety and effectiveness have not been established.
  • In people with high blood pressure and healthy kidneys, hydralazine tends to increase blood flow through the kidneys, improving kidney function. However, it should be used with caution in people with advanced kidney damage.
  • Oral tablets are available as 10mg, 25mg, 50mg, and 100mg strengths. Hydralazine 37.5mg is also available in fixed combination with isosorbide dinitrate 20mg.
  • Hydralazine oral tablets are only available as a generic.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • A headache, nausea, vomiting, diarrhea, heart palpitations, and weight loss. Less common side effects include constipation, dizziness, shortness of breath, changes in blood counts, nasal congestion, and flushing.
  • Because hydralazine stimulates cardiac muscle, it may cause chest pain, angina attacks, and ECG changes. Associated with a higher risk of heart attack; therefore, should not be used in people with coronary artery disease.
  • Requires regular monitoring of BP during initiation.
  • Not a preferred agent for the management of hypertension. Usually used as add-on therapy if BP not adequately controlled with guidelines recommended antihypertensive agents, such as ACE inhibitors, angiotensin II receptor antagonists, calcium-channel blockers, or thiazide diuretics.
  • Historically was considered the agent of choice for the management of hypertensive emergencies associated with pregnancy (such as preeclampsia or eclampsia); however, IV labetalol may be preferred for its more rapid onset, shorter duration of action, and more predictable hypotensive effect.
  • May not be suitable for some people including those with a history of stroke or rheumatic heart disease. Not recommended for severe hypertension or hypertensive emergencies associated with stroke or in people with cerebral edema and encephalopathy.
  • Avoid abrupt discontinuance in people with a marked reduction in BP caused by hydralazine; reduce dosage gradually.
  • May cause damage to peripheral nerves, leading to weakness, numbness, and pain in the hands and feet. The addition of pyridoxine (vitamin B6) may help.
  • Hydralazine is one of the most common drugs implicated in Drug-induced Systemic Lupus Erythematosus (DILE). Symptoms may occur anywhere from three weeks to two years after taking hydralazine and include rash, fever, weight loss, fatigue, joint or muscle pain, and kidney inflammation. Seek urgent medical advice if this occurs.
  • The oral tablets require 2 to 4 times daily administration.
  • May interact with some drugs including MAOI antidepressants and other antihypertensives.
  • Hydralazine should be avoided during the first trimesters of pregnancy and its use is not recommended during the third trimester unless its benefits outweigh its risks. Animal studies have shown hydralazine is associated with birth defects, including cleft palate and malformed facial and cranial bones at higher doses. Hydralazine is also excreted into human milk; however, use is considered acceptable if the benefits outweigh the risks.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

Hydralazine lowers blood pressure (BP) but also stimulates the heart which may increase the heart rate and lead to angina symptoms. It is not a preferred agent for BP-lowering but may be used in addition to other recommended treatments in people whose BP is hard to control. Hydralazine is one of the most common drugs associated with drug-induced systemic lupus erythematosus (DILE).

5. Tips

  • May be taken with or without food, but should be taken consistently either with or without food. Food may increase the blood levels of hydralazine, increasing the risk of side effects such as a blood pressure drop when standing.
  • Be cautious driving or performing tasks that require concentration if hydralazine makes you dizzy. Also, be careful when going from a sitting or lying down to a standing position. Alcohol, dehydration, exercise, or illness may exacerbate these effects.
  • Tell your doctor if you experience any chest pain or heart palpitations while taking hydralazine.
  • Seek urgent medical advice if you develop a rash, fever, joint or muscle pain, difficulty breathing, or numbness, or nerve pain in your hands or feet.
  • Talk to your doctor or pharmacist before buying other medications over the counter to check that they are compatible with hydralazine.
  • Tell your doctor if you are pregnant or plan to become pregnant or breastfeed before taking hydralazine.

6. Response and effectiveness

  • Peak blood levels of hydralazine are usually reached one to two hours after oral hydralazine; sooner if injectable hydralazine is used.
  • Effects are short-lasting, necessitating a four-times daily dosing regimen.
  • 20–25 mg of IV hydralazine hydrochloride is approximately equal to 75–100 mg of oral hydralazine hydrochloride.
  • BP-lowering effect occurs in 20–30 minutes following oral administration, 10–30 minutes following IM administration, and 5–20 minutes following IV administration.
  • BP-lowering effect lasts for 2–4 hours following oral administration or 2–6 hours following IM or IV administration.

7. Interactions

Medicines that interact with hydralazine may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with hydralazine. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with hydralazine include:

  • amphetamines, such as dexamphetamine
  • antipsychotics, such as haloperidol or thioridazine
  • beta-blockers (eg, atenolol, sotalol): additive hypotensive effect, usually used to therapeutic advantage
  • diazoxide
  • diuretics, such as furosemide or HCTZ: additive hypotensive effect, usually used to therapeutic advantage
  • duloxetine
  • epinephrine
  • levodopa
  • methylphenidate
  • monoamine oxidase inhibitors, such as isocarboxazid or phenelzine
  • NSAIDs such as diclofenac or ibuprofen
  • other vasodilators, such as nitroglycerin
  • pholcodine
  • phosphodiesterase 5 inhibitors, such as sildenafil and tadalafil (when used in combination with hydralazine/isosorbide dinitrate)
  • selegiline.

Note that this list is not all-inclusive and includes only common medications that may interact with hydralazine. You should refer to the prescribing information for hydralazine for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use hydralazine only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: May 23, 2022.